An active Atlantic hurricane period coming

The remnants of Tropical Depression Five are no longer a threat, done in by high wind shear and close proximity to land. However, an active period for Atlantic hurricanes is likely for the remainder of August, as the global atmosphere undergoes a major change to the circulation pattern that has dominated Northern Hemisphere weather during July and August. A large trough of low pressure is gathering strength over Europe, and is expected to push eastward. By Thursday, this trough should be able to push away the blocking ridge of high pressure that has given Russia its worst heat wave in history. The shift in circulation has already weakened the large region of sinking air that has brought dry, stable conditions to the tropical Atlantic over the past month. Vertical instability, which was unusually low since late July, has now returned to near normal levels over the tropical Atlantic (Figure 1), though it remains quite low over the rest of the North Atlantic. Instability is measured as the difference in temperature between the surface and the top of the troposphere (the highest altitude that thunderstorm tops can penetrate to.) If the surface is very warm and the top of the troposphere is cold, an unstable atmosphere results, which helps to enhance thunderstorm updrafts and promotes hurricane development. Since SSTs in the Atlantic were at record highs and upper tropospheric temperatures were several degrees cooler than average in July, enhancing instability, something else must have been going on to reduce instability. Dry air can act to reduce instability, and it appears that an unusually dry atmosphere, due to large-scale sinking over the Atlantic, was responsible for the lack of instability. Now that vertical instability has returned to near normal levels, Atlantic hurricane activity should increase to at least average levels over the next two weeks. This is particularly true since SSTs are at record highs and vertical wind shear is at average to below average levels over the tropical Atlantic.

Figure 1. Vertical instability (in °C) over the Caribbean (left) and tropical Atlantic between the Lesser Antilles Islands and coast of Africa (right) in 2010. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere became much more stable than normal in both regions at the end of July. This lack of instability also extended to the Gulf of Mexico and North Atlantic Ocean between Europe and North America. However, in the past few days, vertical instability has returned to normal, thanks to a major pattern shift in the global atmosphere. Image credit: NOAA/CIRA.

Figure 2. The climatology of Atlantic hurricane activity shows a sharp rise in activity around August 18.

AnalysisAugust 18 historically marks the point where Atlantic hurricane activity makes a major spike upwards (Figure 2.) On average, we can expect to see two named storms and one hurricane during the last half of August. The last half of August usually sees a moistening of the atmosphere off the coast of Africa, as the the African monsoon kicks into high gear. This year is no exception (Figure 3.) The dry Saharan Air Layer (SAL) has retreated to the north, leaving a moist atmosphere conducive for tropical cyclone development off the coast of Africa.

It would not be a surprise to see atmospheric instability increase to above-average levels by early next week as the major atmospheric pattern shift progresses. Will this usher in a hyperactive period of Atlantic hurricane activity next week, with a parade of three or four simultaneous storms strung out across the Atlantic? Probably not, since the Madden-Julian Oscillation (MJO) only marginally favors upward motion over the tropical Atlantic, and is not forecast to change much over the next ten days. The Madden-Julian oscillation is a pattern of enhanced rainfall that travels along the Equator from west to east. The pattern has a wet phase with large-scale rising air and enhanced thunderstorm activity, followed by a dry phase with large-scale sinking air and suppressed thunderstorm activity. Each cycle lasts approximately 30 - 60 days. When the Madden-Julian oscillation is in its wet phase over a hurricane-prone region, the chances for tropical storm activity are greatly increased. The bottom line: I expect we will see 2 - 3 named storms in the Atlantic by the end of August, including one hurricane. Where these storms might develop and move is difficult to say. It currently appears that the global shift in circulation will bring near-average steering currents to the Atlantic over the next ten days, with a series of troughs of low pressure capable of recurving hurricanes, moving across the Atlantic. The GFS model is indicating, though, that during the few days of August, these troughs may weaken, making recurvature of hurricanes less likely, and increasing the probability of landfalling storms.

The GFS, NOGAPS, and ECMWF currently predict that one or two tropical storms will form between Africa and the Lesser Antilles Islands during the period 4 - 10 days from now. The NOGAPS model is predicting the development of a strong tropical disturbance near the coast of Honduras late this week.

Figure 3. Saharan Air Layer (SAL) analysis shows that the dry air and dust of the SAL (orange colors) lies well to the north of the hurricane breeding grounds off the coast of Africa, near the Cape Verdes Islands. Image credit: University of Wisconsin/NOAA Hurricane Research Division.

Smoke bedeviling Moscow againLight easterly winds over the past few hours have brought smoke from wildfires back into Moscow today. Temperatures at Moscow's Domodedovo airport hit 29°C (84°F) today, which is 11°C (20°F) above average. The latest forecast for Moscow predicts that just one more day remains for Russia's greatest heat wave in recorded history. On Thursday, a strong trough of low pressure will move through European Russia, finally bringing below average temperatures.

Quoting troy1993:Levi32..when exactly is this Cape verde development is suppose to take place? I have been hearing that the tropical wave currently over Africa might merge with the one off of Africa..what is your take on this?

It would seem the models got a little confused for a while there and may have picked the wrong wave. That one is heading mostly north and out into the dry air of the northeast Atlantic, and it's entirely possible that the wave following close behind it will be the real catalyst for any development. However it happens, I do feel the situation is ripe for Cape Verde development, and model support for it is impressively strong.

Quoting wunderkidcayman:the TWO slow development possible due to unfavored environment but that could and might change in 48>hours when its in the W carib

Unlikely. Upper level conditions are forecast to remain unfavorable for development. Convection is also being maintained by diffluent flow aloft. Likely that nothing will come of it. Personally, PGI31L should be the one circled, not that area (PGI27L).

Levi32..when exactly is this Cape verde development is suppose to take place? I have been hearing that the tropical wave currently over Africa might merge with the one off of Africa..what is your take on this?

Quoting wunderkidcayman:KANKUNKID WHAT DID YOU JUST SAY DID I JUST HEARD YOU CALL MY HOME A PIECE O' LAND YOU KNOW I KNOW SOME PEOPLE THAT WOULD STICK A BULLIT IN YOU BUTT FOR SAYING THAT BUT I AM A NICE GUYS SO I AM NOT GOING TO GO OVERBOARD WITH THIS JUST REMEMBER MOST PEOPLE LOVE THERE HOME LAND OK

Don't think he meant it in a bad way , he knows most storms that affect us usually come around his area too!

Sorry Alaina to truncate your post. I am a NWS trained spotter and I must have fell asleep during the water over the road part. It makes me wonder what the UN-trained spotters would have reported..Anyway, can you imagine the class when they train them how to tell whether there is water over the road?"Now that thar roads got water ALL over it. This'n ain't got none.."The other question is how are you supposed to checked the depth of "unknown waters" so they become known? Your buddy's automobile?I know it's not supposed to be funny, but it cracked me up. I apologize in advance to the sensitive..

I was so wondering the same thing!! LOL. So since im not smart enough to pass the maths needed to be a met im thinking I could become a trained weather spotter instead? LOL... sounds good to me.